Conventionally, pellets are formed by adding a filler and, if desired, compounding additives such as a stabilizer, a pigment, etc. to a crystalline polyolefin. Heat-melting and kneading are then carried out on the resulting mixture to make pellets. The pellets are then used in molding processing.
However, there are problems in obtaining pellets containing a large amount of filler in the above-described manner, because the filler generally has a finer particle size and smaller bulk density than a crystalline polyolefin powder. Examples of these problems are described below.
(1) a blend of a crystalline polyolefin powder and a filler separate from each other can cause segregation of the particles in a hopper; or when transporting the blend from hopper to hopper non-uniform dispersion of the filler can result.
(2) The blend causes bridging in a hopper or is poor in feeding with a screw of an extruder (or a kneader); this prolongs the plasticizing time, resulting in a series decrease in pelletizing ability or makes it impossible to carry out pelletization.
These defects become more serious as the amount of filler added increases. Therefore, in pelletizing polyolefin containing a large amount of filler, special consideration must be given with respect to the structure of the hopper, and that of the feeder for the blend of filler and crystalline polyolefin and that of the extruder screw. Accordingly, costly equipment is required. In addition, as the amount of filler increases, the resulting pellets, if produced, contain filler that is not uniformly dispersed which leads to poor appearance and poor mechanical properties of resulting moldings.
Thus, it has been extremely difficult, using prior art techniques, to produce granules containing a large amount of filler, particularly granules containing enough filler to be used as a master batch, by heat-kneading and subsequent pelletizing.
Japanese Patent Publication No. 45612/76, and U.S. Pat. Nos. 3,962,531, 3,997,494, 4,045,403, and 4,046,849 propose a process for producing a granular composition containing a comparatively large amount of solid powder in a thermoplastic resin without conducting the above-described pelletizing.
Japanese Patent Publication No. 45612/76 describes a process for producing a granular composition containing a thermoplastic resin and solid powder as major components. The process comprises adding a molten thermoplastic resin to a zone where the solid powder is in a fluid state under the action of strong stirring. However, this process requires an independent thermoplastic resin-melting system and a quantitative spraying means in addition to the mixing and stirring equipment.
Japanese Patent Publication No. 8228/79 describes a process for compounding a thermoplastic polymer material and a filler. The process comprises blending thermoplastic polymer particles with a particulate filler, gradually melting the surface portion of the thermoplastic polymer particles to thereby adhere and deposit the particulate filler onto the thus molten surface portion, and separating the thus obtained intimate mixture of the molten surface portion and the filler from the polymer particles. However, this process requires a narrow temperature range for gradually melting the polymer material, and is extremely difficult to control. In addition, in this process, the weight ratio of filler to polymer is at most 2 as described in the specification, which is quite different from that in the present invention. This is attributed to the fact that, as the amount of filler is increased, poor dispersibility of the filler results and, upon molding, there is serious deterioration of mechanical properties.
Furthermore, control of particle size and particle size distribution are so different in the above-described two processes that particles having uniform particle size are difficult to obtain.
The present inventors have carried out intensive investigations for obtaining granules containing a large amount of uniformly dispersed filler as a major component, which granules have a uniform particle size distribution and which can be used as a master batch. As a result the present inventors have found a process for obtaining granules containing a large amount of filler with crystalline polyolefin powder as nuclei, i.e., granules comprising a filler, crystalline polyolefin poweder, a binder having a melting point lower than that of the crystalline polyolefin powder and, if necessary, predetermined compounding additives.